(1) 2NO + O2 → 2NO2 (Brown)
(2) Copper is placed below hydrogen in electrochemical series and does not liberate H2 from acid. However, HNO3 oxidises copper due to its strong oxidant nature.
3Cu + 8HNO3(dil.) → 3Cu(NO3)2 + 4H2O + 2NO
(3)Pb(NO3)2 + Δ → N2O4 + Δ → NO2 ↑ (Brown)